Green Manures booklet - Institute of Organic Training and Advice

4.5.3. Slugs
There are instances of increased slug populations in subsequent crops as the result of a grass clover ley being
sown (Frank, 1998). As slugs are considered one of the major pests in organic vegetable crops (Peacock &
Norton, 1990), it is essential that the green manure does not increase their numbers. A trial investigating the
effect of different fertility building crops on slug populations (IACR- Long Ashton, 2002) came up with the
following recommendations:
• For short term cover crops grown to prevent nutrient leaching, ryegrass resulted in less severe slug problems
in the following crops compared to legumes such as clover or vetch.
• Where a leguminous fertility building crop is required, Lucerne resulted in slower growth of the slug
population than other popular legumes such as clovers or vetch.
It is important to note that this work was only carried out at one site and further work needs to be done as slug
numbers vary with soil type, crop and season.
4.5.4. Wireworms
Wireworms (Agriotes spp) are another pest that can be harboured by green manures or grass clover leys. They
are the larvae of the click beetle and even relatively low populations can cause large reductions in marketable
yields of potato crops. In the UK, high numbers of wireworms have traditionally been associated with long grass
clover leys (Miles, 1942, Anon 1948) especially those longer than one year (Schepl & Paffrath, 2005). The only
way to ensure that wireworm damage does not occur in potato crops is to test the soil and to grow the crop
somewhere else if it is present. Levels can be monitored in the field but seemed to be an unreliable indication as
to the level of damage that will be inflicted on the subsequent potato crop (Parker, 1996). Therefore tests can
only be used as a simple indicator as to the presence or absence of the pest. Some measures can also be taken to
ameliorate the effects of fertility building crops on wireworm populations. Schepl & Paffrath (2005) suggested
that peas or lupins may be a better precrop to potatoes than field beans or red clover. Good weed control was
also deemed important as perennial weeds such as creeping thistle and docks were found to increase numbers of
the pest. Autumn ploughing followed by disking when wireworms are most active in the upper layers of the soil
is more likely to have an effect on reducing wireworms than ploughing in the spring (Gratwick, 1989). The
possibility of using brassica green manures such as mustard which contain glucosinilates as a biofumigant
against wireworms was tested by Frost (2002). In this trial, effects on wireworm populations were negligible but
it may be that other varieties with higher levels of glucosinilates may be more effective.
4.5.5. Leatherjackets
Leatherjackets are the larvae of craneflies (Tipula spp.) and have the potential to cause extensive damage in
spring cereals, potatoes, leeks and lettuces following a ley (Soil Association, 2005). They will also result in loss
of production of the ley itself. The most extensive damage occurs between September and mid June when the
larvae feed on the roots of crops (White & French, 1968). Surveys of grassland (Blackshaw, 1984) showed
populations ranging from 65 000 to 865 000 /ha and predicted losses 50 kg/ha of dry matter at populations of
125 000 / ha. Some measures are known to reduce the incidence of this pest (e.g. keeping ley tightly cut between
July and September to reduce the chances of adults laying eggs) but more work is needed to investigate methods
of control.
4.5.6. Bean seed fly
The bean seed fly (Delia platura), also known as the seed corn maggot, is another potential pest that may be
influenced by the presence of green manures. It is a small dipterous insect that has a wide host range of up to 40
different crop types (Valenciano et al., 2004). It attacks the cotelydons of germinating seeds as they emerge and
can cause extensive stand loss. Ploughing in organic matter from green manures may provide a favourable
environment for egg laying and timing of planting in relation to tillage can have a key effect on the amount of
damage caused (Hammond & Cooper, 1993; Hammond, 1995). This can lead to an ongoing problem with
further generations and will be addressed within Project FV 299. Hammond and Cooper (1993) suggested that
eggs are laid at the time of tillage and that damage could be reduced by delaying planting until after the feeding
maggots had pupated. Observations from this work suggested a period of 250 day degrees (above a base of
3.9°C) until pupation although it would be prudent to take this as a rough guide as it is highly likely that other
factors will affect the life cycle of the pest.
The use of green manures
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